1. Field of the Invention
This invention relates to new binders for two-component polyurethane coating compositions containing a polyisocyanate component and a hydroxy-functional liquid polyacrylate component having an at least bimodal molecular weight distribution.
2. Description of the Prior Art
Two-component polyurethane coating compositions based on hydroxy-functional polyacrylates (the term "polyacrylate" means polymers and copolymers of acrylic or methacrylic acid esters with themselves or with other monomers) and polyisocyanates are known (see e.g. Glasurit-Handbuch Lacke und Farben, 11th Edition, pages 66 et seq, Publishers Curt R. Vincentz Verlag, Hanover, 1984). They are distinguished by outstanding gloss retention, UV resistance and chemical resistance and are superior in these respects to two-component polyurethane coating compositions based on polyesters or coating compositions containing mixtures of polyacrylate copolymers with polyesters as the hydroxyl component. Coatings with good hardness and scratch resistance as well as flexible coatings can be obtained with suitable choice of the comonomers. The polyhydroxy polyacrylates conventionally used are compatible and miscible with the polyisocyanates used as cross-linking agents.
Comparatively high molecular weight polyhydroxy polyacrylates are generally used for the preparation of two-component polyurethane lacquers based on hydroxy-functional polyacrylates which dry quickly at room temperature. Due to their high molecular weight, these polyhydroxy polyacrylates have a high viscosity and therefore are generally used at high dilutions with solvents to ensure that the coating compositions have a sufficiently low working viscosity. Such laquer systems are described, e.g., in EP-A-0 068 383.
Due to ever shorter cycle times, in particular in industrial processes, there is at present a great demand for two-component polyurethane coating compositions which can be dried even more rapidly than the systems of the above-mentioned publication and which for ecological reasons do not require an increase in the proportion of of organic solvents.
It is therefore an object of the present invention to provide new binder mixtures for two-component polyurethane coating compositions based on polyisocyanates and hydroxy-functional polyacrylate resins which dry considerably more rapidly at room temperature but do not have a higher viscosity or higher solvent content.
This object may be achieved by the binder mixtures according to the invention described below. An essential feature of the invention is the observation that binder mixtures based on organic polyisocyanates and hydroxy-functional polyacrylates in which the polyacrylate component has an at least bimodal molecular weight distribution are suitable for solving the stated problem.